This invention relates to a digital oscilloscope apparatus having means for visually displaying electrical waveforms.
Oscilloscopes of a conventional analog type and more recently of a digital type are widely employed in the measurement and analysis of changing physical phenomena. Generally, a transducer develops an analog voltage signal in accordance with a sensed physical phenomena. The transduced electrical signal is connected as the input to the oscilloscope which includes beam control means for providing a visual display or representation of such signal as a waveform. The changes being measured and analyzed may be either continuous and periodic or transient. In modern oscilloscopes, a trigger circuit is included so that the portion of the transduced signal displayed is that portion which occurs at a specific selectable time following a trigger event. In a conventional analog oscilloscope, a CRT tube includes a scanning beam which sweeps across a screen to generate the display. In the analog oscilloscope the CRT sweep starts when the trigger signal arrives. Thus, the portion of the waveform displayed is that immediately following the trigger signal and continuing for one horizontal full scale sweep time. In the case of a transient (one time only) signal, the conventional oscilloscope operates in a single sweep mode wherein the instrument is triggered once and sweeps once to present the signal. In the case of a continuous and periodic signal, the conventional oscilloscope is run in a mode whereby as soon as the CRT sweep resulting from one trigger signal is completed, the instrument automatically enables its trigger circuit to accept another trigger signal and repeat the process. Since the signal is periodic, if the trigger signal is derived from a point on the waveform itself the effect is that each sweep of the CRT display is written over the previous one and the displayed waveform representation appears stationary. If the input periodic waveform is, however, not perfectly periodic in that each succeeding period differs slightly from the last, then the display, which is a succession of individually displayed periods of the signal, varies from sweep to sweep, showing the changing character of the waveform much as a motion picture projector shows individual frames, which differ slightly, in rapid succession to give the overall impression of a varying or moving picture.
The above mentioned trigger event may be a special signal pulse whose main purpose is to trigger the oscilloscope but more often it is a part of the input signal waveform which has certain requisite characteristics. The oscilloscope is provided with user operated controls for selection of the source and the particular characteristic for generation of the trigger event. The particular characteristic may be a voltage level, a voltage rate of change, a voltage direction of change or the like. The characteristic which is to form the trigger event is specified by the user by adjustment of the controls which are usually set by changing the control settings while watching the resulting changes on the displayed waveform. Thus, the usual method of adjusting oscilloscope trigger controls requires that the displayed waveform immediately reflect the effect of the adjustment to the user.
The conventional oscilloscope, in its several modes, directly processes and displays the input signal upon the screen for a momentary finite time, after which the signal is lost. If any of the data or information is to be retained for extended viewing, photographic means must be provided, the signal stored in some other medium or a special type of CRT possessing a mechanism for storage of images must be employed.
The more recent digital type oscilloscopes employ a finite digital memory in which the waveform input signal is stored and from which the display is generated. The digital oscilloscope thus performs the function of displaying a representation of the input waveform by digitizing the input signal, storing such digitized signal in memory and reproducing a visual representation of the analog signal from such digitized data. Various modes of signal acquisition and presentation are provided in recently developed digital oscilloscopes. The waveform can be presented as a continuous live display in which the input waveform appears to flow across the display screen, being created at the right-hand edge of the screen as it occurs, or the flow may be halted and one full memory array thus captured for a static display. Display and analysis of a periodic continuous waveform with the digital oscilloscope usually is accomplished by providing sequence sweep signals allowing the instrument to take one full memory sample of the waveform which is displayed and then a new full sample taken and again displayed.
Recent developments of data taking in a digital instrument known as a transient recorder provide for a triggered mode of capturing the desired portion of the changing signal by selection of its time relationship with the trigger signal. In such a mode the data acquisition is coupled to the memory means until the triggering event occurs, and then such coupling is continued until a specified delay time expires, after which the data acquisition is halted by decoupling from the memory means. The waveform in the memory is then the waveform as existing at the time specified with relation to the trigger event.
The triggered mode of operation was initially developed for measuring and recording transient phenomena which permit the user to capture and analyze a transient waveform. Hence the devices are generally known as transient recorders. The transient recorder may be coupled to a suitable oscilloscope to display the captured transient signals in much the same way as the conventional oscilloscope. That is, a trigger signal produces a single data memory array containing a single portion of the input waveform.
A particularly unique waveform measuring and analyzing instrument incorporating all of the characteristics of digital oscilloscopes and transient recorders and, in addition, programmed signal processing capability is disclosed in the copending application of Frederick A. Rose entitled "WAVEFORM MEASURING MEANS WITH RESIDENT PROGRAMMED PROCESSOR FOR CONTROLLED WAVEFORM DISPLAY AND WAVEFORM DATA REDUCTION AND CALCULATION", filed Ser. No. 670,703 on the same day as this application and assigned to the same assignee, now U.S. Pat. No. 4,072,851. As more fully disclosed therein, the instrument is allowed to free run, that is, to continuously take samples of the waveform and shift them through memory in the manner of a transient recorder. This free running mode produces the effect of the live waveform moving from the right edge to the left edge on the display screen. For slow sampling rates, useful for slowly varying signals, the waveform marches slowly across the screen and variances in successive periods can easily be perceived. For faster varying signals the sample rate, and, hence the rate of marching, must be increased and eventually a point is reached where the eye cannot follow or discern detail in the individual periods of the waveform, and the display becomes a rather useless blur. The addition of a trigger circuit in the instrument permits the operator to capture a single period or portion of the waveform, without the need to discern detail on the display. However, the display of the one sample portion, once captured, is completely static and does not show slow variations similar to a motion picture effect, as does the conventional oscilloscope.